Method and apparatus for measuring lateral variations in thickness or refractive index of a transparent film on a substrate

ABSTRACT

A method or apparatus for measuring lateral variations of a property such as thickness or refractive index of a transparent film on a semiconductor, in a region of repeated patterning comprises: illuminating over the patterned area with a beam of light of multiple wavelengths, the beam having dimensions to include repeated patterning; detecting the intensity of light reflected over the patterned area for each wavelength; producing a signal defining the variation of the intensity of the detected light as a function of the wavelength of the detected light; decomposing the signal into principal frequencies thereof, determining from the principal frequencies, values of the thickness etc. of the transparent film; and applying the values to repetitions within the repeated patterning.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for measuringthe thickness or the refractive index of a transparent film on asubstrate. The invention is particularly useful in measuring thicknessvariations of a photoresist film on a semiconductor substrate, and istherefore described below with respect to this application.

A typical semiconductor manufacturing process uses more than 17photolithography steps. In each step, a photoresist (PR) material isdeposited on the semiconductor (e.g., silicon wafer) surface, and anoptical process is used to copy patterns onto the PR. The patterned PRis then used as a masking layer for subsequent process steps, such asetching, implanting, depositing, scribing, grinding, etc.

A photolithography process includes the following steps:

a) film, wherein the PR layer is coated uniformly over the wafer,

b) baking, wherein the PR is baked at a moderate temperature to dry thesolvents therein;

c) exposing, wherein the wafer is exposed through a mask in which therequired pattern appears as a non-transparent printing;

d) developing, wherein a chemical process is applied to remove theexposed PR from the wafer, and.

e) post exposure baking, wherein the wafers are baked after the exposurein order to harden the photoresist.

The photolithography process is one of the most challenging technologiesused in the semiconductor industry. The patterns printed in the criticallayers set the dimensional limitations for the entire technology. Theminimum line width achieved today in production is 0.18 μm (1/1,000 mm);the next generation of technologies is expected to require minimum linewidth of 0.1 μm and below.

To achieve the minimum line width with high uniformity within each waferand from wafer to wafer, it is most important to control each and everyparameter in the photolithography process. Exposure energy, PR chemicalcomposition, development time and baking temperature are only a few ofthe parameters that can affect the final critical dimension (CD) and itsuniformity.

One of the most critical parameters to be controlled is the PRthickness. Because of the optical nature of photolithography,fluctuations of several nano-meters in the layer thickness can have asubstantial effect on the final CD.

In the current processes, the PR film is applied on a high speedspinning chuck, called a “spinner”. The wafer is placed on the spinningchuck and is rotated at low speed while the PR is dispensed at thecenter of the wafer. After dispensing, the chuck is rotated at a highspeed (300-5000 RPM). The centrifugal forces acting on the PR cause thePR liquid to flow towards the edge of the wafer. Most of the PR (ca.95%) is spilled off the wafer and is collected in a bowl to be drainedlater. The adhesion forces between the wafer surface and the PR hold asmaller amount of the PR on the wafer. The final thickness is a functionof the centrifugal forces, the adhesion to the surface, and the shearforces caused by the viscosity of the liquid. The viscosity is increasedduring the spin due to the solvents evaporation therefore the solventsevaporation rate affects also the final thickness.

To control the final thickness one should control the rotational speed,the ramp-up speed (“acceleration”), the PR viscosity, and theenvironmental conditions within the bowl, among other parameters.

After spinning, the wafer is transferred to sit on a hot plate toperform the pre-exposure bake. Solvent evaporation during the bakefurther reduces the PR thickness. Nonunifornity of the hot platetemperature can cause nonuniformity of the final thickness.

Today, the process is set up and controlled by running a bare silicontest wafer and measuring the final thickness, and thickness uniformity,by a stand-alone layer-thickness measurement device. In order to ensureprocess stability, a periodic test is done on the test wafer. If thethickness drifts out of the control limits, the production is stoppedand corrective actions are taken to bring the final thickness back totarget.

The above-described thickness monitoring procedures are inefficient andwasteful. They can cause serious delays in reacting to a process out ofcontrol (OOC)-Test wafers and PR, as well as operational and engineeringtime, are spent in running the tests in a stand-alone system. Machineoperational time is also wasted when the processing of production wafersis held up until positive test results are obtained. If the test resultsare negative, reworking may be required, or a complete batch may have tobe scrapped.

Moreover, in contrast to the test wafers used for measurement purposes,the real production wafers have a much more complicated topography belowthe PR layer. As can be seen from a comparison of FIG. 1a to FIG. 1b,there may be a large difference in the thickness at different places(d1-d2), which differences are not reflected in a flat test wafermeasurement.

In addition, since most of the PR ends up in the bowl, a considerablequantity of PR material is wasted. PR is one of the most expensivematerials in the semiconductor process and will probably be even moreexpensive when deep-UV lithography is implemented in future productionprocesses.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved method andapparatus for measuring the lateral variation of the thickness of atransparent film on a production wafer. Another object is to provide amethod and apparatus which are particularly useful in measuring lateralthickness variations in photoresist films on semiconductor substrates toavoid many of the above described drawbacks in the existing method andapparatus.

According to one broad aspect of the present invention, there isprovided a method of measuring lateral variations of a property,selected from the group consisting of thickness and refractive index, ofa transparent film on a substrate, including the steps of: (a)illuminating the film with a beam of light of multiple wavelengths; (b)detecting the intensity of the light reflected from the transparent filmfor each wavelength; (c) producing a signal defining the variation ofthe intensity of the detected light as a function of the wavelength ofthe detected light; (d) decomposing the signal into principalfrequencies thereof; and (e) determining from the principal frequenciesthe lateral variations of the property of the transparent film.

According to another broad aspect of the present invention, there isprovided an apparatus for measuring lateral variations of a property,selected from the group consisting of thickness and refractive index, ofa transparent film on a substrate, including: (a) an illuminating devicefor illuminating the film with a beam of light of multiple wavelengths;(b) a detector for detecting the intensity of the light reflected fromthe transparent film for each wavelength; and (c) a processor for: (i)producing a signal defining the variation of the intensity of thedetected light as a function of the wavelength of the detected light,(ii) decomposing the signal into principal frequencies thereof, and(iii) determining, from the principal frequencies, the lateralvariations of the property of the transparent film.

According to another aspect of the present invention, there is provided,in a process for fabricating integrated circuits on a wafer, whereintrenches are formed on a surface of the wafer and the surface then iscovered with an oxide layer that fills the trenches, a method ofmeasuring depths of the trenches, including the steps of: (a)illuminating at least a portion of the oxide layer with a beam of lightof multiple wavelengths; (b) detecting the intensity of the lightreflected from the oxide layer for each wavelength; (c) producing asignal defining the variation of the intensity of the detected light asa function of the wavelength of the detected light; (d) decomposing thesignal into principal frequencies thereof; and (e) determining thedepths of the trenches from the principal frequencies.

According to further features in the preferred embodiment of theinvention described below, the film is a transparent coating on asemiconductor substrate, preferably on a die among a plurality of diescarried on a semiconductor wafer. As will be described below, a numberof advantages are obtained when the beam of light is large enough tocover a complete die.

According to another aspect of the present invention, there is providedapparatus for measuring the thickness of a transparent film on thesubstrate in accordance with the above method.

As will also be described below, the method and apparatus of the presentinvention are particularly useful for measuring thickness variations orrefractive index variations of a film in situ, and in real time, e.g.,to control the deposition or removal of a layer on a semiconductorsubstrate.

Further features and advantages of the invention will be apparent fromthe description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1a is a schematic illustration of a PR layer on top of a flat testwafer,

FIG. 1b is a schematic illustration of a PR layer on top of a patternedwafer,

FIG. 2 is a diagram illustrating the reflectance diffraction of a thinlayer, which diagram will be referred to in describing the basic theoryof the present invention;

FIG. 3a is a schematic layout of one embodiment of an apparatus of thepresent invention;

FIG. 3b is a schematic layout of another embodiment of an apparatus ofthe present invention;

FIGS. 4a and 4 b are detailed views of the apparati of FIGS. 3a and 3 b,respectively;

FIGS. 5a and 5 b are detailed top views of the apparati of FIGS: 3 a and3 b, respectively;

FIG. 6 is a flow chart of the measurement sequence in the apparatus ofFIG. 3a;

FIG. 7 is a simulated reflection signal for a two thickness case;

FIG. 8 is the analyzed results of the signal of FIG. 7;

FIG. 9 is a real reflection signal from a wafer with oxide on top ofmetal lines and intermetal dielectric;

FIG. 10 is the analysis of the signal of FIG. 9;

FIG. 11 is an electronic microscope cross section of the wafer measuredin FIG. 9;

FIG. 12 is a flow chart of the signal processing procedure; and

FIG. 13 is a detailed flow chart of the pre-processing and processingstages in FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Basic Theory of Operation

As indicated above, the method and apparatus of the present inventionare particularly suitable for measuring the thickness of a transparentfilm, particularly a photoresist coating, where there are differences inthickness at different places of the substrate, as illustrated in FIG.1. Thus, FIG. 1 illustrates two different layer thickness d₁-d₂ in atypical wafer. These differences in thickness would not be accuratelymeasurable in a conventional measurement. The method and apparatus ofthe present invention, however, are capable of measuring such thicknessvariations in applied or removed, in situ and in real-time. Thefollowing description of the basic theory of operation of the presentinvention will be helpful in understanding how this is done.

Several methods are known for measuring thickness of laterally uniformtransparent films using the reflected pattern of multi-wavelength light.Thus, as shown in FIG. 2, when a monochromatic (single wavelength) lightbeam arrives at a laterally uniform transparent film, part of the beamis reflected from the upper face (Layer 0/Layer 1 interface), and partis reflected from the bottom face (Layer 1/Layer 2 interface).

FIG. 2 illustrates the above, wherein:

λ is the wavelength of the light;

φ₀ is the phase angle of the incident light (and of the light reflectedfrom the Layer 0/Layer 1 interface):

φ₀+φ₁ is the phase angle of the light reflected from the Layer 1/Layer 2interface, φ₁ being the phase difference associated with propagation ofthe light through Layer 1;

r₀₁ is the reflection coefficient of the Layer 0/Layer 1 interface;

r₁₂ is the reflection coefficient of the Layer 1/Layer 2 interfaces; and

I is the intensity of the incident light

I=I ₀ cos(2πct/λ+φ ₀)  (EQ. 1)

Where I₀ is the intensity amplitude and c is the speed of light

For light arriving perpendicularly to the film surface, the reflectioncoefficient from the top and bottom surfaces are:

r ₀₁=(n ₁ −n ₀)/(n ₁ +n ₀) r ₁₂=(n ₂ −n ₁)/(n ₂ +n ₁)  (EQ. 2)

wherein n₀, n₁, n₂ are the refractive indices of layers 0,1 and 2,respectively.

The light reflected from the upper face interferes with the lightreflected from the bottom face, giving a reflection coefficient (R)which is a function of the layer thickness and the layer refractiveindex. This reflection can be described by the, well-known, Fresnelequation.

R=(r ₀₁ ² +r ₁₂ ²+2r ₀₁ r ₁₂ cos 2Φ₁)/(1+r ₀₁ ² r ₁₂ ²+2r ₀₁ r ₁₂ cos2Φ₁)  (EQ. 3)

where:

Φ=2πn ₁ d ₁/λ  (EQ. 4)

d₁—layer thickness.

Illumining the film with multi-wavelength light, and measuring thereflectance at each wavelength (λ), will give R as a function of λ,i.e., R(λ).

Illuminating a product wafer having a complex (i.e., laterally varying)topography with a large spot of multi-wavelength light will cause areflected beam which is a composition of the separate reflection of eachof the thicknesses alone.

R(λ,d ₁ , . . . ,d _(n))=Σ_(i) (r _((i−1)i) ² +r _(i(i+1)) ²+2r_((i−1),i) r _(i(i+1)) cos 2Φ_(i))/(1+r _((i−1),i) ² r _(i(i+1)) ²+2r_((i−1),i) r _(i(i+1)) cos 2Φ_(i))  (EQ. 5)

By simple mathematical operations it is possible to express thereflection coefficient by:

R(1,d ₁ , . . . d _(n))=Σ_(i)[1−A/(1+B Cos(2Φ_(i)))]  (EQ. 6)

Where

A _(i)=(1−r _((i−1),i) ²)(1−r _(i(i+1)) ²)/(1+r _((i−1),i) ² r _(i(i+1))²)

B _(i)=2r _((i−1),i) r _(i(i+1))/(1+r _((i−1),i) ² r _(i(i+1)hu 2))

Applying ways of frequency decomposition of the reflection coefficientcan give each of the arguments (Φ_(i)) and from equations 3 & 4 we candetermine the laterally varying layer thickness, assuming that we knowthe layer refractive index, or the laterally varying layer refractiveindex, if we know the layer thickness.

There are several ways to perform the frequency decomposition. Some ofthen are suggested below:

Mathematical decompositions

1) The family of orthogonal transform methods, for example, Fouriertransforms

2) The family of methods based on the maximum likelihood principle

3) The family of methods based on Parametric models

4) The family of subspace decomposition methods (cf. Prabbakar S. Naidu,Modern Spectrum Analysis of Time Series, CRC Press, 1996)

Electrical decomposition:

1. Filtering Method

Electrical frequency filters are widely used in electrical systems.Those filters act as window in the frequency domain and output theamplitude of the component of the input signal at the specifiedfrequency of the filter. Passing the reflected signal (translated intoelectrical signal) through a set of filters or a single filter withvariable frequency will give the desired decomposition.

2. Correlation Method

This method is a search of significant correlation coefficients during aserial or parallel process correlation between the reflected signals anda set of pure sinusoidal signals.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIGS. 3-5 schematically illustrate two set-ups which may be used formeasuring the thickness variations of a transparent layer on asemiconductor substrate in accordance with the present invention. Inthis example we describe a set-up of the system mounted on a Photoresistcoating track. One setup is typical to the spinning bowl and the otheris typical to the hot plate. The first set-up includes a spinner system2, (FIG. 3a), comprising a spinner chuck 3 for receiving the wafer W,and a motor 4 having an encoder 5, for rotating the chuck, and the waferthereon, while a photoresist applicator 6 (FIG. 4a) dispenses thephotoresist material at the center of the wafer. The wafer W is firstrotated at a low speed as the photoresist material is dispensed at itscenter, and then is rotated at a high speed (e.g., 300-5000 rpm) byelectric motor 4, which produces centrifugal forces causing thephotoresist liquid to flow towards the edge of the wafer W. Most of thephotoresist (e.g. about 95%/) is spilled off the wafer and is collectedin a bowl 1 (FIG. 4) to be drained later, while the adhesion forcesbetween the wafer surface and the photoresist bold smaller amounts ofthe photoresist as a coating 8 on the wafer. As briefly describedearlier, the final thickness of the photoresist coating 8 is producedwhen a balance is achieved between the centrifugal forces, the adhesionto the surface, and the shear forces caused by the viscosity of thephotoresist liquid. During the spinning process the solvent contained inthe photoresist evaporates and the viscosity increases. Therefore thefinal thickness is also a function of the solvents evaporation ratewhich is affected by the temperature, air flow and other environmentalconditions.

The second set-up describe the hot plate used for further drying of thephotoresist following the spin-coating step. In this module the wafer isplaced on top a hot plate (9 in FIG. 3b) and left there, for previouslyset time, for further solvent evaporation of the solvents contents ofthe photoresist The evaporation causes further reduction of thephotoresist thickness.

As illustrated in FIGS. 3-5, the apparatus further includes anilluminating device for illuminating the photoresist coating 8 with abeam of light of multiple wavelengths, preferably infrared, visible orultraviolet light, and a detector for detecting the intensity of thelight reflected from the photoresist coating 8 for each wavelength As isknown in the art, the range of wavelengths used should be selected tomatch the expected lateral variation of the thickness of photoresistcoating 8.

The illuminating device is schematically shown at 10 in FIG. 5. Itapplies a beam of multispectral light in any suitable manner, e.g., viaan optical fiber 11, to an optical head 12 mounted above wafer W toproject a beam of light 13 onto the photoresist coating 8 of wafer W aswafer W is rotated. The light reflected from the photoresist coating 8is directed in any suitable manner, e.g., via another optical fiber 14,to a spectrum analyzer 15 for detecting the intensity of the lightreflected from photoresist coating 8 for each wavelength.

In order to get information on the thickness or refractive index of morethen one spot on the wafer, multiple optical heads 12 can be placed atdifferent locations above the wafers (FIGS. 3b 4 b & 5 b). The probeheads are connected to a multiplexer unit 19. This unit can direct eachof the beads separately to the spectrum analyzer 15, byelectromechanically switching between each of the optical fibers.Similarly, optical fiber 11 leads to a light splitter 21 which directsthe multispectral light from light source 10 to multiple optical heads12. This setup enables data acquisition from different sites on thewafer.

Spectrum analyzer 15 is schematically illustrated in FIG. 3 as being ofthe type including a photodiode array. Such an array includes amicro-grating that splits the light beam to its spectral components anda photodiode detector for each wavelength. The outputs of the photodiodearray will be in the form of electrical signals representing theintensity of the light reflected from the photocoating for eachwavelength.

The outputs of spectrum analyzer 15 are fed to a processor 16 whichprocessors these outputs according to the basic theory of operationdescribed above, and displays the outputs on a screen 17. In addition,an output of processor 16 may also be used for controlling theapplication of photoresist coating 8 onto wafer W, as shown by thefeedback line 18 from processor 16 to the motor 4 of the spinner system.eg. for controlling its rotational speed to control the centrifugalforces applied to the photoresist coating. It will be appreciated,however, that the output from processor 16 could be used for otherfeed-back controls, e.g., for controlling the rate of application of thephotoresist coating to the wafer via applicator 6, or the hot platetemperature.

As shown schematically in FIGS. 4 and 5, optical head 12 is mounted, viaan arm 20, for movement along the radial axis, to enable the opticalhead to be located at any selected radius of the wafer W under test.During the wafer rotation the angular position of wafer W is identifiedby the encoder 5. This, combined with the radial position, gives thelocation of the measurement spot on wafer W. In the second examplemultiple optical heads 12 are mounted on fixed mounting arms to monitorthe thickness or refractive index on the wafer below the positions whereoptical heads 12 are located (FIG. 5b). Preferably, the beam of light 13is large enough to cover at least one complete die of the plurality ofdies carried by wafer W, as schematically shown in FIG. 3. The use ofsuch a beam of light large enough to cover a complete die, or a multiplethereof provides a number of advantages. Thus, it better assures thatthe combined reflected light detected by detector 15 will not changesubstantially between measurements, irrespective of the difference inthe exact measurement position. Further, the large spot size increasesthe signal collected by the detector and also increases the speed ofdetection. Using a large spot size is contrary to the current trends inlayer thickness measurement systems which systems use small spot sizesin order to detect different thicknesses.

The flow chart of FIG. 6 illustrates one manner of using theabove-described apparatus for measuring the thickness of the photoresistcoating 8 in situ at the time of application of the photoresist coating,and also for controlling the application of the photoresist coating. Asshown in FIG. 6, the wafer W is first loaded on the spinner chuck 3(block 30). In some of the application a prior knowledge of thethickness of the sublayer is required and will be measured prior to thePR dispense (block 31). Photoresist material is dispensed from theapplicator 6 at the center of the wafer (block 32), as the wafer isrotated by the spinner motor 4 (block 33). An initial measurement of thephotoresist coating is made (block 34), and subsequent measurements areperiodically made (block 35), under the control of the rotary encoder 5of the spinner motor 4. When the desired thickness is obtained, asdetermined by processor 16, processor 16 terminates the operation ofspinner motor 4, via feed-back line 18 (FIG. 3a). Similar procedure canbe applied in the case of the hot plate (FIG. 3b). Processing thereflected signal from the different probe heads gives the thickness orrefractive index information from each measurement sites on the wafer.Feeding the information back to the hot plate temperature controller andto the system timer causes real-time feed back on the process and can beused as end-point timer for the baking step.

FIG. 7 illustrates a simulation of the above-described process whenapplied to a wafer coated with a photoresist coating of two thicknesses(d₁, d₂). FIG. 7 thus illustrates the sum R of the reflectancecoefficient R₁ and R₂ as a function of thickness d₁ and d₂ respectivelyand the wavelength In this example d₁=0.95μ;d₂=1.25μ, and ƒ (the ratioof the intensity of the separated signals)=1.

FIG. 8 illustrates the results of applying a Fourier Transformation tothe signal R defining the total reflection. This Transformation producesa series of Fourier Coefficients for the signal frequencies, from whichthe thickness of the transparent film can be determined for eachrespective signal frequency. As shown in FIG. 8, the two thicknesses ofthe photoresist coating produced two peaks, each representing aprincipal frequency related to one of the thicknesses.

FIG. 9 illustrates a measured reflection signal from anotherapplication, in this measurement a wafer with pattern of metal lines wascovered by an inter-metal dielectric on silicon dioxide (Oxide). Twothicknesses of oxide are present within the measurement spot: 1) oxideon top of the metal lines, with d₂=1556 nm, and oxide on top ofpreviously deposited dielectric of thickness of d₁=722 nm (FIG. 11).FIG. 10 is the signal received after processing by a frequencydecomposition process similar to the one presented in FIGS. 12 and 13.The results show the peaks at a frequency related to d₂ and d₁+d₂.

The flow chart of FIG. 12 illustrates one manner of analyzing theoptical signals in above-described apparatus for measuring thethickness/refractive index of a transparent layer. Prior to the actualmeasurement the system has to be set-up (Block 40) and calibrated. The‘dark signal’ is the background signal collected in the system withoutreflected light (Block 41). The collection of the dark current can bedone by placing a fully absorbing plate bellow optical bead 12. The darksignal is stored in the computer memory. The ‘Reference signal’ is thesignal collected in the system from a perfectly reflecting surface(Block 44). This signal can be acquired by placing a mirror or a barepolished silicon wafer below the optical head. The ‘Reference signal’ isalso stored in the computer memory (Block 44). All the steps 41-44 canbe done only periodically, as required, and do not have to be repeatedbefore each measurement.

For the actual measurement the wafer has to be placed on its stage 4below optical head 12 (Block 45). The reflected signal is collected inthe system (Block 46) and the reflection coefficient is calculated(Block 47) using:

R(λ)=(Signal(λ)−Dark(λ))/(Reference(λ)−Dark(λ))

Where R(λ) is the reflection coefficient as a function of thewavelength, Signal(λ) is the reflected signal from the wafer,Reference(λ) is the reference signal stored in the computer memory andDark(λ) is the background signal stored in the computer memory.

The signal preprocessing (Block 48) is composed of the following steps:Filtering the signal from its low frequency components (lock 4S1 in FIG.13). Those frequencies are parasitic. Frequency decomposition (Block482) is done using the methods mentioned above. Only selectedfrequencies are of interest and the fine detailed search is done withinpreviously selected frequency windows. (Block 483)

The signal processing (Block 49) is composed of the following steps: Thesignificant peak frequencies are found (Block 49-1) and the frequency atits maximum is identified (Block 49-2). Calculating the refractive indextimes thickness is performed (Block 49-3) using equation 4. If therefractive index is previously known then the exact thickness can befound and if the thickness is previously known then the refractive indexcan be found (Block 50). The results are then sent to the computer(Block 51) to be used by the engineers or a feed back data to theprocess.

Preferably, the measurements should be taken while the wafer is spinningat typical speeds of 3000-5000 r.p.m. For a light spot at the edge of a300 mm diameter wafer, the linear velocity then is about 70 m/sec. Thephotoresist thickness on the wafer changes gradually along tenth of mm,therefore an averaged result collected within a 10 mm-20 mm arc issufficient In order to receive a measurement resolution of 20 mm, themeasurement time should be about 0.3 millisecond.

In order to achieve the required speed, it is preferably to use aparallel spectrometer detector 15. Such a spectrometer splits the lightto its different wavelengths and directs them to the photodiode array,thereby measuring the intensity of each wavelength in parallel. Addingthe detected signals during several cycles can reduce the speed demandof the measurement by a factor of 10/30, and can improve thesignal/noise ratio. One of the ways to ensure the collection of the datafrom the same spot over several rotations can be to illuminate the waferwith a stroboscopic light synchronized to the rotation speed andilluminating the wafer every time the same spot passes under the opticalprobe. The photoresist thickness may be changing at a rate of tenths ofseconds, allowing enough margin for multi-cycle signal collection Therotary encoder 5 of spinner motor 4 would perform tracking of the waferangular position

The above-described method and apparatus may also be used forcontrolling the removal of a transparent film, e.g., by a reactive gas,by a plasma process, by mechanical polishing (for example, chemicalmechanical polishing (CMP)), etc. The removal rate of a film can becontrolled by continuously monitoring the thickness of the film in themanner described above during the removal process.

The above-described method and apparatus may also be used forcontrolling the refractive index of a layer such as low-K dielectric. Inapplications where the thickness uniformity and repeatability are wellwithin the expected limits, any change of the frequency (fn) is a resultof change of the refractive index, which in turn gives valuableinformation on the layer porosity.

One important application of the present invention is to monitoring thedepths of isolation trenches in integrated circuits (IC).

The basic units of ICs are MOS transistors. In a typical IC, largenumbers of transistors are printed close to each other. The everincreasing demand for smaller and smaller dies leads IC designers toreduce the gap between the transistors, which increases the chance ofelectrical leakage between the transistors.

Special techniques have been developed to improve the electricalisolation between the transistors. One of these includes the steps ofetching relatively deep trenches between the transistors and filling thetrenches with silicon dioxide. The process of filling the trenchesincludes the steps of depositing a thick layer of silicon dioxide on thewafer and polishing and etching the silicon dioxide down to the surfaceof the wafer, leaving silicon dioxide only inside the trenches.

The present invention allows the depths of the silicon-dioxide-filledtrenches to be measured at any point in the process subsequent to thedeposition of the silicon dioxide layer, by measuring the thickness ofthe silicon dioxide layer at the trenches and away from the trenches. Inparticular, the present invention enables accurate control of theremoval of the silicon dioxide down to the level of the trenches, andalso enables measurement of the uniformity of the depths of thetrenches.

Other applications of the present invention will be apparent, forexample, in the application or removal of inter-metal dielectric layers,applied between two metal layers on a semiconductor substrate, or anyother transparent layer, such as poly-imide. The invention could also beused in the application or removal of films on printed circuit boards orflat panel displays.

Further variations, modifications and applications of the invention willbe apparent to those skilled in the art.

What is claimed is:
 1. A method of measuring lateral variations of aproperty, selected from the group consisting of thickness and refractiveindex, of a transparent film, in a patterned area comprising repeatedpatterning, on a substrate, comprising the steps of: (a) illuminatingover the patterned area with a beam of light of multiple wavelengthssaid beam having dimensions to include repeated patterning within anillumination area; (b) detecting the intensity of the light reflectedover the patterned area for each wavelength; (c) producing a signaldefining the variation of the intensity of the detected light as afunction of the wavelength of the detected light; (d) decomposing saidsignal into principal frequencies thereof; (e) determining, from saidprincipal frequencies, values of the property of the transparent film assaid property varies laterally over said repeated patterning and (f)applying said values to repetitions of said property within saidrepeated patterning.
 2. The method according to claim 1, wherein saidfilm is a transparent coating on a semiconductor substrate.
 3. Themethod according to claim 2, wherein said semiconductor substrate is adie among a plurality of dies carried on a semiconductor wafer.
 4. Themethod according to claim 3, wherein said patterned area is large enoughto cover at least one complete die.
 5. The method according to claim 1,wherein the intensity of an interference pattern of the light reflectedfrom the upper and lower surfaces of the transparent film is detectedfor each wavelength.
 6. The method according to claim 1, wherein theintensity of the light reflected from the transparent film is detectedby a spectrum analyzer having a photodiode detector-for each wavelength.7. The method according to claim 1, wherein the property of thetransparent film is measured in situ in real time with the applicationof the transparent film to the substrate; and the property measurementis used to control the application of the transparent film to thesubstrate.
 8. The method according to claim 1, wherein the transparentfilm is a photoresist film applied to a semiconductor substrate duringthe processing of the semiconductor substrate.
 9. The method accordingto claim 1, wherein the transparent film is an intermetal dielectriclayer applied to a semiconductor substrate between two metal layers todielectrically isolate the two metal layers.
 10. The method according toclaim 1, wherein the property of the transparent film is measured insitu in real time with the removal of the transparent film from thesubstrate, and the property measurement is used to control the removalof the transparent film from the substrate.
 11. The method according toclaim 8, wherein the property of the transparent film is measured insitu in real time with the development of the photoresist, and theproperty measurement is used to monitor the development rate of thephotoresist.
 12. The method according to claim 9, wherein the propertyof the transparent film is measured in situ in real time with thedevelopment of the photoresist, and the property measurement is used tomonitor the development rate of the photoresist.
 13. The methodaccording to claim 10, wherein the property of the transparent film ismeasured in situ in real time with the development of the photoresist,and the property measurement is used to monitor the development rate ofthe photoresist.
 14. The method of claim 1, wherein said wavelengths areselected from the group consisting of infrared wavelengths, visiblewavelengths and ultraviolet wavelengths.
 15. The method of claim 1,wherein said decomposing is effected using an orthogonal transformationmethod.
 16. The method of claim 1, wherein said decomposing is effectedusing a maximum likelihood method.
 17. The method of claim 1, whereinsaid decomposing is effected using a method based on a parametric model.18. The method of claim 1, wherein said decomposing is effected using asub-space frequency decomposition method.
 19. The method of claim 1,wherein said decomposing is effected by steps including: (i)transforming said signal into an electrical signal, and (ii) filteringsaid electrical signal.
 20. The method of claim 19, wherein saidfiltering is effected using a narrow pass filter with a variable centralfrequency.
 21. The method of claim 19, wherein said filtering iseffected using a plurality of narrow pass filters of successively highercentral frequencies.
 22. Apparatus for measuring lateral variations of aproperty, selected from the group consisting of thickness and refractiveindex, of a transparent film in a patterned area on a substrate,comprising: (a) an illuminating device for illuminating the patternedarea over a region comprising repeated patterning with a beam of lightof multiple wavelengths, said beam being of such dimension as toencompass said repeated patterning; (b) a detector for detecting theintensity of the light reflected from the transparent film for eachwavelength; and (c) a processor for: (i) producing a signal defining thevariation of the intensity of the detected light as a function of thewavelength of the detected light, (ii) decomposing said signal intoprincipal frequencies thereof, and (iii) determining, from saidprincipal frequencies, values of the property of the transparent film assaid property varies laterally over said repeated patterning and (iv)applying said values to repetitions of said property within saidrepeated patterning.
 23. The apparatus according to claim 22, whereinsaid detector includes a spectrum analyzer for separating the reflectedlight into its different wavelengths, and a photodiode array including aphotodiode detector for each wavelength.
 24. The apparatus according toclaim 22, wherein said substrate is a die among a plurality of diescarried on a semiconductor wafer, and said patterned area is largeenough to cover a complete die.
 25. The apparatus according to claim 22,wherein said detector detects the intensity of an interference patternof the light reflected from the upper and lower surfaces of thetransparent film for each wavelength.
 26. The apparatus according toclaim 22, wherein said apparatus further comprises: (d) a support forsupporting said substrate.
 27. The apparatus according to claim 26,wherein said apparatus further comprises: (e) a rotary drive forrotating said support while the substrate thereon is illuminated by saidilluminating device, and while the intensity of the reflected light isdetected by said detector.
 28. The apparatus according to claim 26,wherein said support is a hot plate.
 29. The apparatus as claimed inclaim 22, wherein the apparatus further includes a film applicator forapplying said transparent film to the substrate, and means forcontrolling said film applicator in real time in response to themeasured thickness of the transparent film.
 30. The apparatus accordingto claim 29, wherein said film applicator applies a photoresist film ona semiconductor substrate.
 31. The method of claim 22, wherein saidwavelengths are selected from the group consisting of infraredwavelengths, visible wavelengths and ultraviolet wavelengths.
 32. Theapparatus according to claim 22, wherein said illuminating deviceincludes: (i) an optical head, and (ii) a mechanism for moving saidoptical head to scan the transparent film with said beam of light. 33.The apparatus according to claim 22, wherein said illuminating deviceincludes a plurality of optical heads at fixed positions relative to thetransparent film.
 34. In a process for fabricating integrated circuitson a wafer, wherein trenches are formed on a surface of the wafer andthe surface then is covered with an oxide layer that fills the trenches,a method of measuring depths of the trenches, comprising the steps of(a) illuminating at least a portion of the oxide layer with a beam oflight of multiple wavelengths; (b) detecting the intensity of the lightreflected from the oxide layer for each wavelength; (c) producing asignal defining the variation of the intensity of the detected light asa function of the wavelength of the detected light; (d) decomposing saidsignal into principal frequencies thereof; (e) determining from saidprincipal frequencies a first layer depth away from said trenches, and asecond layer depth at said trenches, and (f) deriving from said firstand said second layer depths a trench depth.